1. Field of the Invention
The present invention relates to a high-strength steel sheet having excellent workability and, specifically, to a high-strength steel sheet which is excellent in balance between strength and total elongation (especially local elongation in the latter stage of transformation out of total elongation) and has a low yield ratio. More specifically, there is provided a steel sheet which satisfies strength [TS (MPa)]×local elongation [l-EL (%)]≧5500, ratio [(l-EL)/(t-EL)] of local elongation to total elongation [t-EL (%)]≧0.25, and yield ratio (YR)≦65%.
2. Description of the Related Art
Demand for high-strength steel sheets is growing, mainly backed by efforts to reduce fuel cost by reducing the weight of a steel sheet for automobiles and ensure safety at the time of a collision. That demand is further growing from the viewpoint of the preservation of the global environment.
However, moldability is strongly desired even for high-strength steel sheets and it is important to select and use a high-strength steel sheet having suitable moldability according to application purpose. Particularly in application fields in which steel sheets are pressed to a complex shape, the provision of a high-strength steel sheet which has both stretch-formability (ductility) and stretch-flange properties (hole expandability (local ductility)) (that is, low yield ratio) and good balance between strength and local elongation is earnestly desired.
As a high-strength thin steel sheet which has been developed to meet the above need is known the residual austenite steel sheet in which the residual austenite is formed in the structure and ductility is improved by the induction transformation (transformation-induced plasticity: TRIP) of YR during processing deformation. For example, Japanese Laid-open Patent Application No. 2-97620 discloses a TRIP composite structure steel (PF steel) composed of a mixed structure of ferrite, bainite and the residual austenite. According to the above publication, the steel is produced by heating at a bainite transformation temperature range and maintaining that temperature for a predetermined time (so-called austempering). Therefore, C having a large dispersion constant is concentrated in untransformed austenite and stabilized to cause austenite to remain without transforming it into martensite at room temperature, thereby obtaining a high-strength steel sheet having excellent workability. However, nowadays when importance is attached to both ductility and workability, further improvement of ductility (especially local elongation) is strongly desired.
Japanese Laid-open Patent Application No. 5-255799 discloses a steel sheet which contains one or more out of bainite, martensite and the residual austenite and ferrite and its local ductility is much higher than a conventional TRIP steel sheet. However, when the yield ratio of Example is calculated, it is 78% or more which means the steel sheet is inferior in stretch-formability. The reason for this seems to be that the amount of the formed martensite and the like useful for the reduction of yield ratio greatly decreases because the top priority is placed on the improvement of ductility (especially local elongation) in the above steel sheet.